Introduction, Background, and Principal Results of Leg 128 of the Ocean Drilling Program, Japan Sea

Kodaira

et al. 2014

JGR Solid Earth

To clarify the formation process of the back-arc basin in the Japan Sea, which is located next to the northwestern Pacific, a seismic survey using ocean bottom seismographs and an air gun array was undertaken in areas from the northern Yamato Basin to the coast of the northeastern Japan Island Arc off Awa-shima Island. The crust beneath the northern Yamato Basin off Awa-shima Island is approximately 16 km thick. The upper and lower crusts are, respectively, about 5 km thick with a steep velocity gradient and about 10 km thick with a gentle velocity gradient. In the basin, there are very few units with P wave velocity of 5.4-6.0 km/s, corresponding to the continental upper crust. The crustal structure of the northern Yamato Basin has characteristics of thicker oceanic crust. The high-velocity lowermost crust in the northern Yamato Basin with 7.2-7.4 km/s might show melt formed by a slightly high mantle temperature during the opening of the basin. However, the crust beneath the areas from the Sado Ridge to the coast, which is approximately 25-26 km thick, is slightly thinner than that of the continental crust and island arc crust. The crustal structure beneath this area is inferred to be a rifted continental and/or a rifted island arc crust.

Seismic constraints of the formation process on the back‐arc basin in the southeastern Japan Sea

Kodaira

et al. 2014

JGR Solid Earth

Transition from continental rift to back-arc basin in the southern Japan Sea deduced from seismic velocity structures

Geophysical Journal International

Arai

et al. 2020

SUMMARY We obtain the crustal structure from active-source seismic surveys using ocean bottom seismographs and seismic shots to elucidate the evolutionary process from continental rifting to the backarc basin opening in the Yamato Basin and Oki Trough in the southern Japan Sea. Results show that the crust changes from approximately 14–15 km thick in the basin (the southern Yamato Basin) to 16.5–17 km in the margin of the basin (the southwestern edge of the Yamato Basin). The P-wave velocity distribution in the crust of the southern Yamato Basin is missing a typical continental upper crust with P-wave velocities of 5.4–6.0 km s–1, and is thought be a thicker oceanic crust formed by a backarc basin opening. By contrast, the crust of the southwestern edge of the Yamato Basin might have been formed by continental rifting because there is an unit with P-wave velocities of 5.4–6.0 km s–1 and with a gentle velocity gradients, corresponding to the continental upper crust in this area. This variation might reflect differences in mantle properties from continental rifting to backarc basin opening of the Yamato Basin. Because the Oki Trough has a crustal thickness of 17–19 km and having a unit with P-wave velocities of 5.4–6.0 km s–1, corresponding to the continental upper crust with a high-velocity lower crust, we infer that this trough was formed by continental rifting with magmatic intrusion or underplating. These crustal variations might reflect transitional stages from continental rifting to backarc basin opening in the southern Japan Sea.

Characteristics of crustal structures in the Yamato Basin, sea of Japan, deduced from seismic explorations

Kodaira

2023

Island Arc

Crustal structures around the Yamato Basin in the southeastern Sea of Japan, inferred from recent ocean bottom seismography (OBS) and active‐source seismological studies, are reviewed to elucidate various stages of crustal modification involved from rifting in the crust of the surrounding continental arc to the production of oceanic crust in the Yamato Basin of the back‐arc basin. The northern, central, and southern areas of the Yamato Basin have crustal thicknesses of approximately 12–16 km, and lowermost crusts with P‐wave velocities greater than 7.2 km/s. Very few units have P‐wave velocities in the range 5.4–6.0 km/s, which corresponds to the continental upper crust. These findings, combined with previous geochemical analysis of basalt samples, are interpreted to indicate that a thick oceanic crust has been formed in these areas of the basin, and that this oceanic crust has been underplated by mantle‐derived magma. In the central Yamato Basin, the original continental crust has been fully breached and oceanic crust has been formed. Conversely, the presence of a unit corresponding to the continental upper crust and the absence of a high‐velocity part in the lower crust implies that the southwestern edge of the Yamato Basin has a rifted crust without significant intrusion. The Oki Trough has a crust that is 17–19 km thick with a high‐velocity lower crust and a unit corresponding to the continental upper crust. The formation of the Oki Trough resulted from rifting with magmatic intrusion and/or underplating. We interpret these variations in the crustal characteristics of the Yamato Basin area as reflecting various instances of crustal modification by thinning and magmatic intrusion due to back‐arc extension, resulting in the production of a thick oceanic crust in the basin.